When you open the editor, press f3 and then at the top right select options and then load. Navigate through the single player folders and access the Overlord mission. Once in the folder, select D-Day.

Once loaded, press f1 and then select start at the bottom right. Now press Numpad 1 to take control of the Allies.

Note:

This requires Men of War: Assault Squad and will not run in the original Men of War.
Also, due to the recent Assault Squad patch I was forced to make tiny changes to this mission to ensure it remains compatible. Therefore, the latest patch for the game is advisable (may not run without it).

This requires a very powerful rig. Even on an overclocked i7 CPU, I struggle to run this smoothly sometimes!